The present invention relates to a device for generating pressure variations of large amplitude in an acoustic tube without forming shock waves in the fluid contained in the acoustic tube.
Already known as pressure wave generators in a gas fluid are acoustic compressors which comprise an acoustic tube with an inlet and an outlet for the fluid provided at one end thereof, and a driving device connected to the other end of the acoustic tube [see, for example, JP-A No. 11-303800(1999), No. 8-219100(1996), No. 4-224279(1992), etc.]. The driving device produces pressure variations within the acoustic tube which cause the fluid to be discharged from the tube through the outlet while taking the fluid into the tube through the inlet. The fluid is compressed by a pressure difference between in the intake fluid and discharge fluid.
In the conventional acoustic compressors, however, shock waves generally appear as the pressure variations become large. This not only imposes limitations on the magnitude of amplitude of pressure variations in the fluid to limit the pressure difference between in the intake fluid and discharge fluid, i.e., the compression ratio of the fluid, but also causes heat generation in the fluid, and accordingly in the compressor itself, to a high temperature and producing loud noise.
An object of the present invention is to produce shock-free pressure variations of larger amplitude than the ones produced by conventional acoustic compressors or like pressure wave generators.
The present inventors previously made a theoretical analysis of the propagation of nonlinear acoustic waves in a tunnel provided with an array of Helmholtz resonators (xe2x80x9cPropagation of nonlinear acoustic waves in a tunnel with an array of Helmholtz resonators,xe2x80x9d J. Fluid Mech. (1992), vol. 244, pp. 55-78). In consequence, the inventors found that the shock wave which emerges from the pressure waves generated by entry of a high-speed train into the tunnel can be effectively suppressed by a suitable array of Helmholtz resonators connected to the tunnel, as arranged axially thereof. The present inventors have conceived the idea of applying this theory to the suppression of shock waves in pressure wave generators, and substantiated the effect thereof to accomplish the present invention.
The present invention provides a pressure wave generator which comprises a closed acoustic tube 1, and a driving device 3 generating vibration mounted as directed toward the interior of the tube 1 at the resonance frequency of a fluid in the acoustic tube 1 or at a frequency close to the resonance frequency, a plurality of Helmholtz resonators 2 each having a channel which connects the cavity of the resonator with the interior of the acoustic tube 1 and being arranged on a periphery of a tubular wall of the acoustic tube 1 with suitable axial spacing (see FIG. 1).
Each of the Helmholtz resonators 2 comprises a throat 21 having a narrower channel in diameter than the acoustic tube 1 and joined at a base end thereof to the tubular wall of the acoustic tube 1, and a closed cavity of suitable volume 22 joined at one end of the throat.
As will be apparent from the experimental result to be described later, the pressure wave generator of the present invention produces the same effect (geometrical dispersion as will be described later) as that involved in the propagation of pressure waves in the tunnel provided with an array of Helmholtz resonators, whereby the generation of shock waves in the tunnel is suppressed.
Stated more specifically, the acoustic tube 1 connects thereto an intake pipe 13 and a discharge pipe 14, whereby an acoustic compressor is provided, in which the gas taken in through the intake tube 13 is compressed and discharged from the discharge pipe 14.
According to another specific embodiment, the acoustic tube 1 is in the form of a straight tube or loop, and a regenerator 41 in the form of stack of flat plates or a porous regenerator 44 disposed inside the acoustic tube 1, and the channel of the acoustic tube 1 is provided with a high-temperature heat exchanger 42 and a low-temperature heat exchanger 43 (see FIG. 8) connected respectively to the high-temperature end and the low-temperature end of the regenerator 41 or 44, whereby an acoustic refrigerator is provided to execute radiation and absorption of heat through the two heat exchangers 42, 43.
The Helmholtz resonators 2 can be replaced by a plurality of closed side-branch 2a from the acoustic tube 1 serving as the resonators (see FIG. 11). Usable as the driving device 3 in place of a linear motor is a device wherein a plate 32 as attached to bellows 31 is driven to reciprocate by a piezoelectric vibrator 35, or a device wherein a diaphragm 36 is driven instead of the bellows (see FIG. 10).
The pressure wave generator of the present invention has a simple construction wherein an array of cavities is arranged along an acoustic tube and by which generation of shock waves is effectively suppressed, generating a shock-free and larger pressure amplitude than the ones conventionally available entailing generation of shock waves.